Dahlias should fit the bill. One caveat is that you have to dig them
up for winter, or buy new tubers each spring. If you divide them
after digging up, you get an exponential growth in the number of
plants each year though.

Nice piece of advice. By the way, you can check for a great selection of Dahlia Tubers at "http://www.lynchcreekdahlias.com/"